The invention relates to materials useful for bone tissue repair.
There have been a number of materials studied to initiate bone repair and/or to restore or replace missing bone to address the problem of stimulating formation of bone at specific sites.
Among the approaches used to address this problem is a conformational method whereby an implant material, usually made of metal ceramic or other inorganic material in a form intended to mimic the form of the missing bone, is inserted into the site in which bone replacement is required. There is a risk that the host will reject the material or there will be a failure of integration of the implant with normal skeletal tissue. Some ceramic materials such as ceramic tricalcium phosphate, although acceptably biocompatible with the host and bone, when used as an implant, appear to lack sufficient mechanical properties of bone for general utility and the bone does not consistently grow into and become incorporated within the implant.
Another approach involves substituting the missing bone tissue with a matrix which functions as a support into which the new bone growth can occur. The theory is that the matrix attracts the cells committed to an osteogenic pathway and the new bone grows in and through the matrix by the process referred to as osteoconduction. Allogeneic bone (non-host bone) grafts are used for this method, however there is a substantially high failure rate. Even when the allogeneic bone grafts are accepted by the host, healing periods for consolidation and capacity for mechanical stress are of comparatively long duration compared to autogeneic bone (host-bone) grafting. The use of allogeneic bone also presents the issue of transmissible viral agents.
A third method involves the process known as osteoinduction, which occurs when a material induces the growth of new bone from the host's undifferentiated cells or tissues, usually around a temporary matrix. A number of compounds are shown to have such a capacity. See for example, U.S. Pat. Nos. 4,440,750 to Glowacki, 4,294,753 and 4,455,256 to Urist and 4,434,094 and 4,627,982 to Seyedin et al. The most effective of these compounds appear to be proteins which stimulate osteogenesis. However, when synthesized from natural sources they are present in extremely low concentrations and require large amounts of starting material to obtain even a minute amount of material for experimentation. The availability of such proteins by recombinant methods may eventually make the use of such proteins per se of more practical value. However, such proteins will probably still need to be delivered to the desired site in an appropriate matrix.
There have been compositions disclosed containing collagen and various forms of calcium phosphate directed to healing and bone growth.
U.S. Pat. No. 5,338,772 to Bauer et al. discloses a composite material containing calcium phosphate ceramic particles and a bio-absorbable polymer where the calcium phosphate ceramic is at least 50% by weight and the particles are joined by polymer bridges. The calcium phosphate ceramic particles are disclosed as having a size of about 20 microns to about 5 mm.
U.S. Pat. No. 4,795,467 to Piez et al. discloses a composition comprising calcium phosphate mineral particles admixed with atelopeptide reconstituted fibrillar collagen. The calcium phosphate mineral particles are disclosed as having a size in the range of 100-2,000 microns.
U.S. Pat. No. 4,780,450 to Sauk et al. discloses a composition for bone repair comprising particulate polycrystalline calcium phosphate ceramic, a phosphophorin calcium salt and a type I collagen in a weight ratio of 775-15:3-0.1:1. The ceramic particles are disclosed as being dense hydroxyapatite about 1 to 10 microns in diameter or larger dense hydroxy apatite ceramic particles of greater than about 100 microns in diameter.
PCT Application WO 94/15653 to Ammann et al. discloses formulations comprising tricalcium phosphate (TCP), TGF-β and, optionally, collagen. The TCP is disclosed as being a delivery vehicle for the TGF-β, such that the TCP is of the particle size greater than 5 microns and preferably greater than about 75 microns. The most preferred range for the size of the TCP granules is disclosed as being 125-250 microns.
PCT Application WO 95/08304 discloses polymineralic precursor particles of hydroxyapatite mixed with insoluble collagen. The particle size of the polymineralic precursor particles are in the range from 0.5 microns to 5 microns. The precursor minerals are converted to hydroxyapatite by hydrolysis, and this process, it is believed, fuses the mineral to form monolithic hydroxyapatite.
British Patent Specification 1,271,763 to FMC Corporation discloses complexes of calcium phosphate and collagen.